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Fair Value Measurements - Additional Information (Details) - USD ($) | 3 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2020 | Jan. 01, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Held-to-maturity securities, at amortized cost | $ 586,870,000 |  | $ 705,838,000 |
Available-for-sale, at amortized cost | 2,431,874,000 |  | 2,210,099,000 |
Available-for-sale securities purchased | 379,760,000 | $ 82,028,000 |  |
Proceeds from sales of available-for-sale securities | 0 | 0 |  |
Held-to-maturity securities purchased | 0 | 0 |  |
Proceeds from sale of held-to-maturity securities | $ 0 | $ 0 |  |
Term of contractual due dates of substantially all mortgage-backed securities (in years) | 10 years |  |  |
Accrued interest for available-for-sale securities | $ 3,997,000 |  | 3,285,000 |
Accrued interest for held-to-maturity securities | 1,506,000 |  | $ 1,811,000 |
Mortgage-backed securities | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Held-to-maturity securities, at amortized cost |  |  |  | $ (374,680,000)
Available-for-sale, at amortized cost |  |  |  | $ 374,680,000
Consumer loans | Single-family residential |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Foreclosed residential real estate properties held as REO | 528,000 |  |  |
Real estate acquired through foreclosure | $ 2,214,000 |  |  |

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